Installer fixes / improvements

This covers two commits so far because for some reason git decided to bring a commit with it when I created the new branch, but hey ho.

First one checks for composer's autoload file before requiring it. This way we can show the user a friendly[ish] message with some guidance on what they need to do. It's fairly basic, but it prevents first impressions of Pyro being a fatal error.

Second part fixes some of the newer Streams code where the ```$where``` stuff was just sitting on its own, not being applied to anything. Eloquent doesn't like arrays (unless there's some undocumented feature I've missed) so I re-jigged the code a bit. This was breaking the installer because when it tried installing the pages streams, it was actually returning the blog stream, meaning the field it needed didn't exist.

If I can get the blog module's install method fixed too (it's using ```Schema::create()``` _and_ adding a blog stream) then this will get the installer running successfully. I haven't really used streams yet but would I be on the right lines thinking the Schema method would need to be converted entirely to use streams' fields model? Unless someone's planning a way of creating a table "manually" with Schema and having Streams manage it afterwards, it's the only way I can think of at least.